1. Field of the Invention
The present invention relates to an image density control method for an electrophotographic type image forming apparatus such as a copier, printer or facsimile device.
2. Description of the Related Art
The demand for improved copier and laser printer image quality in recent years has been simultaneously accompanied by a desire for improved image durability and stability. In other words, there is a need for images that are minimally affected by change when in use (including continuous printing and intermittent printing) and that remain stable over time to be provided. Two-component developer systems in which a two-component developer comprising a non-magnetic toner and magnetic carrier (hereinafter referred to as a developer) that is held on a developer carrier (hereinafter referred to as a development sleeve), and in which development is based on a magnetic brush being formed by housed magnetic poles and the imparting of a developer bias onto the development sleeve at a position opposing a latent image carrier (hereinafter referred to as photoreceptor) have been hitherto widely employed.
These two-developer component systems are widely employed because of the simplicity of color development that they afford. In these systems the two-component developer is carried to a development region accompanying the rotation of the development sleeve. As the developer is being carried to the development region a large number of magnetic carriers in the developer, while aligning themselves with the magnetic lines of force of a developer electrode, aggregate in company with the toner to form a magnetic brush.
Unlike single-component developer systems, in two-component developer systems the precise control of the toner-carrier weight ratio (toner density) is a very important factor in terms of improving stability. For example, when the toner density is too high a soiling of the image skin of a drop in the fine resolution of the image occurs. In addition, low toner density results in an unwanted drop in the density of the solid image part and adherence of the carrier. Accordingly, the toner supply amount must be controlled to adjust the toner density in the developer to the appropriate range.
The toner density control performed here is based on a comparison of an output value of toner density detection means (for example, permeability sensor); Vt and a toner density control reference value; Vref, a calculation of a toner supply amount in accordance with the difference thereof from a calculation formula, and the implementation of toner supply to a development unit by means of a toner supply device.
As the method for detection of toner density a magnetic sensor is normally employed. In this system magnetic permeability changes in the developer produced by changes in the toner density are converted to toner density changes.
Another method of toner density detection employs an optical sensor. This method involves the production of a reference patch on an image carrier or intermediate transfer belt and irradiation of an LED light. The reflected light from the pattern thereof (normal reflected light or diffuse reflected light) is detected by an optical sensor (photodiode or phototransistor or the like) and, based on the result thereof, the toner density (toner adhered amount) is detected.
In another known method for toner density control performed during printing a reference toner pattern is produced between sheets of transfer paper (in the time, or an interval, between when a directly preceding image formation has finished and the forming of the next image is to start), and the toner density control reference value: Vref of a magnetic permeability sensor is successively controlled.
Japanese Unexamined Patent Application No. S57-136667 describes a method comprising means for producing a toner pattern on a non-image part and detecting pattern density and toner density in a development unit in which, in accordance with the density of the toner pattern, image density is maintained by change of a toner density control target value of a development unit.
However, there is a desire for the excessive use of toner that occurs in actual practice when toner patterns are produced between sheets of paper to as far as possible be reduced, and correction based on production of reference toner patterns between sheets of paper is tending now towards an expanding of the interval between the production of the toner patterns, or indeed to not being performed at all.
Furthermore, in the production of toner patterns on an intermediate transfer belt, if the secondary transfer roller is not separated when each individual is formed, a toner cleaning device must be additionally provided to clean the patches of toner between the sheets of paper that adhere to the secondary transfer roller.
In addition, if the secondary transfer roller is separated when each individual image is formed (or several images are formed), while there is no need for a cleaning device to be provided, a mechanical mechanism able to withstand the frequently occurring secondary transfer separation and contact is necessary. For the reason described above, as well as from the viewpoint of reducing the mechanical costs, the toner patterns produced between the sheets of paper must as far as possible be suppressed.
In addition, for example, Japanese Patent No. 3,410,198 discloses, in the implementing of a toner supply control employing a toner density sensor, a method for maintaining the toner density constant by correcting and stabilizing the fluctuations in toner density sensor output produced by changes in the flow state of the developer in accordance with the agitation time.
However, even if a constant toner density is maintained, unless the development capability of the developer is stable, it is difficult to maintain a stable image density by simply keeping the sensor output constant.
In addition, in many of the image formation apparatuses of recent years means for reducing stress in the development device have been incorporated. These methods are regarded as very effective means by which the objects of a lowering of the amount of developer arising because of the demand for the miniaturization of development devices while reciprocally extending the lifespan of the developer are able to coexist. For example, while additives such as silica (SiO2) or titanium oxide (TiO2) are externally affixed (adhered) to most of the surface area of the toner surface in order to improve toner dispersibility in color two-component image forming apparatus, these additives have little resistance to mechanical stress and heat stress. Accordingly, during agitation within the development unit, a phenomenon in which they either become embedded in the toner inner part or separate from the surface thereof occurs and, while changes in the flow and charge characteristics of the developer (including the toner and carrier) and, furthermore, the physical adhesion force between the toner and carrier occur, these phenomena are able to be as far as possible suppressed by these additives.
On the other hand, sometimes the toner charge capability (capability of development unit to change the toner) drops as a result of the lowering of the stress of the development unit. Briefly describing the development process, for example, while the development capability (gradient of a graph in which toner developer amount to developer bias is plotted) is kept constant when an image of low image coverage ratio is output (low toner replacement amount per unit time or unit number of sheets), the development capability increases when an image of high image coverage rate (large toner replacement amount per unit time or unit number of sheets) is output. In other words, differences in development capability occur in accordance with the amount of toner replaced in the developer.
Because, by virtue of this, differences in development capability occur even when the toner density remains unchanged, the toner density in the development unit must be manipulated to the appropriate range by, in order to keep the development capability constant over time, changing the toner density control reference value. Because, as a result, changes in the development capability also occur when the toner density changes, the image forming conditions (development potential) must be set in accordance therewith.
When image forming apparatuses having these characteristics dispense with a conventional composite control comprising a magnetic permeability sensor and a photosensor in which the image density control reference value is changed on the basis of toner patch production on the paper there is a resultant need for the toner density control based on the use of magnetic permeability sensor alone to be implemented more precisely during continuous printing or changing of the image mode. Accordingly, an image density control system to replace the conventional composite control with a photosensor must be adopted.